Syringe assemblies designed for use with replaceable needle assemblies usually contain a luer slip or locking luer type fitting for securing the needle assembly to the syringe barrel. These connections rely on an elongate frusto-conically shaped syringe barrel tip which frictionally engages a frusto-conically shaped passageway in a needle hub. The relatively acute angle of the syringe tip, measured from the longitudinal axis of the syringe barrel, provides an excellent seal between the syringe tip and the hub. However, slight variations in the angle or diameter of the tapered surfaces of the syringe barrel and/or the needle hub produce large variations in the relative position of the needle hub with respect to the end of the barrel and, therefore, variations in dead space. The space inside the barrel tip and between the end of the barrel tip and the end of the cavity in the hub constitutes a dead space containing liquid that cannot be delivered by the syringe.
In the case of expensive medications and multi-patient immunization programs, medication lost in the dead space can become costly. This is especially true in immunization programs involving thousands of people. Controlling dead space to a minimum may result in more people being immunized with the same amount of medication provided for the program.
The prior art also teaches a flat seal, perpendicular to the longitudinal axis of the syringe barrel which mates with a complimentary flat seal on the needle hub for use with a threaded needle assembly and barrel engagement structure. The flat seal reduces the portion of dead space attributable to variations and barrel and hub tolerances. However, unlike the locking luer type fittings that become tighter as additional torque is applied to the needle hub, syringe assemblies having flat and low angle seals reach a sealed condition in a rather abrupt fashion due to their geometry.
When syringes and needle assemblies of any of the above-mentioned geometries are assembled in the manufacturing facility, the needle assembly can be attached to the syringe barrel using an optimal amount of torque applied to the needle hub therefore avoiding needle hubs that are not properly attached which can lead to leaking of medication and needle hubs which are over tightened which can have long term consequences due to creep in the plastic components which relax over time and lose their preload. However, in a clinical setting the attaching of a needle assembly to a syringe barrel is not done with the consistent controlled force found in the manufacturing process. Some users of the syringe and needle assembly are strong, some are weak and many have their own preconceived opinion regarding what the proper torque needed to attach a needle assembly.
Although the prior art teaches many syringe barrel and needle hub connecting structures, there is still a need for a low dead space syringe which will not attach the needle assembly to the syringe barrel unless an adequate amount of torque is applied to the needle hub and which will not tighten further after proper installation has occurred.